Golf ball

ABSTRACT

The present invention provides a golf ball comprising a core, an outermost cover layer and an intermediate layer therebetween. The intermediate layer is formed primarily of a resin material obtained by blending together (I) a sodium ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer with (II) a magnesium ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer. The intermediate layer-forming resin material has a Shore D hardness of from 55 to 70, and the intermediate layer has a thickness of from 0.5 to 2.5 mm. The outermost cover layer is formed primarily of a non-ionomeric resin material. The cover-forming resin material has a Shore D hardness of from 35 to 60, and the cover has a thickness of from 0.5 to 2.0 mm. The golf ball of the invention has a flight performance and controllability acceptable for use by professional golfers and skilled amateurs, and also has an excellent durability to cracking on repeated impact.

BACKGROUND OF THE INVENTION

The present invention relates to golf ball having a core and a cover of one or more layer. More specifically, the invention relates to a golf ball having an excellent rebound and durability.

Recently, preferred use has been made of urethane resin materials in the outermost cover layer. In such urethane balls, to achieve a lower spin rate and a high rebound, the intermediate layer located inside the outermost cover layer is often made of an ionomer resin having a high hardness. Ionomer resins are ionic copolymers of an olefin such as ethylene with an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, in which some of the acidic groups are neutralized with metal ions such as sodium, lithium, zinc or magnesium. In particular, ionomer resins have excellent characteristics such as durability and resilience, and are thus well-suited for use as the base resin in golf ball cover materials.

When a sodium ion-neutralized ionomer is used alone as the intermediate layer material, the ball itself has a good rebound but an inferior durability. When a zinc ion-neutralized ionomer is used alone as the intermediate layer material, the converse is true; that is, the ball itself has a good durability but a poor rebound.

JP No. 3257890 describes a golf ball in which a material obtained by blending a sodium ion-neutralized ionomer with a zinc ion-neutralized ionomer is used to form the intermediate layer.

In addition, art relating to materials obtained by blending magnesium ion-neutralized ionomers with other types of ionomers is disclosed in JP No. 3810133, International Application WO 97/02318, International Application WO 97/02319, U.S. Pat. No. 6,130,296, U.S. Pat. No. 6,712,719 and U.S. Pat. No. 6,746,346.

However, these prior-art materials are all materials having blended therein a terpolymer; they are not materials which give the intermediate layer a high hardness and thus enhance the durability of the golf ball to cracking on repeated impact and the distance traveled by the ball.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golf ball which has a flight performance and controllability acceptable for use by professional golfers and skilled amateurs, and which also has an excellent durability to cracking on repeated impact.

As a result of extensive investigations on ways of achieving the above object, the inventor has found that, in a multi-piece solid golf ball composed of a core encased by a multilayer cover, when the intermediate layer disposed between the core and the outermost cover layer is given a high hardness and the outermost cover layer thereon is made of a non-ionomeric thermoplastic elastomer such as urethane, in the interest of achieving a golf ball having a good flight performance and controllability and also having an improved durability to cracking under repeated impact, it is effective to optimize the material hardness and the thickness of the intermediate layer by using an intermediate layer material composed primarily of a resin material obtained by blending together (I) a sodium ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer with (II) a magnesium ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer.

Accordingly, the invention provides the following golf balls.

-   [1] A golf ball comprising a core, an outermost cover layer and an     intermediate layer therebetween, wherein the intermediate layer is     formed primarily of a resin material obtained by blending     together (I) a sodium ion neutralization product of an     olefin-unsaturated carboxylic acid random copolymer with (II) a     magnesium ion neutralization product of an olefin-unsaturated     carboxylic acid random copolymer, the intermediate layer-forming     resin material having a Shore D hardness of from 55 to 70 and the     intermediate layer having a thickness of from 0.5 to 2.5 mm; and the     outermost cover layer is formed primarily of a non-ionomeric resin     material, the cover-forming resin material having a Shore D hardness     of from 35 to 60 and the cover having a thickness of from 0.5 to 2.0     mm. -   [2] The golf ball of [1], wherein the intermediate layer-forming     material contains material (I) and material (II) in a mixing ratio     by weight of from 20/80 to 80/20. -   [3] The golf ball of [1], wherein the non-ionomeric resin material     in the outermost cover layer is a thermoplastic polyurethane     elastomer.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The golf ball of the invention has a multilayer structure composed of a core and a plurality of cover layers that encapsulate the core. In the invention, the encapsulating layers outside of the core include at least an outermost cover layer and an intermediate layer.

The diameter of the core, while not subject to any particular limitation, is preferably in a range of at least 33.0 mm but not more than 41.0 mm, more preferably at least 35.0 mm but not more than 40.0 mm, and most preferably at least 36.0 mm but not more than 39.0 mm. The core has a deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, while not subject to any particular limitation, is preferably set in a range of at least 2.8 mm but not more than 7.0 mm, more preferably at least 3.0 mm but not more than 6.0 mm, and even more preferably at least 3.2 mm but not more than 5.0 mm. If the core is harder than the above range in values, the feel of the golf ball when played may worsen and, particularly on long shots with a club such as a driver that causes large deformation of the ball, the spin rate may rise excessively, which may keep the ball from traveling as far as desired. On the other hand, if the core is softer than the above range in values, the ball may have a dead feel on impact and insufficient rebound, as a result of which it may not travel as far as desired. Moreover, the durability to cracking under repeated impact may worsen.

The core in the present invention may be provided with a hardness difference between the surface and center of the core, in which case the core surface hardness minus the core center hardness (JIS-C hardness) is generally from 15 to 36, preferably from 18 to 32, and more preferably from 20 to 30. In this way, the spin rate of the ball on full shots can be lowered. If the above hardness difference is too small, the spin rate-lowering effect on shots with a driver (W#1) may be smaller than desirable, shortening the distance traveled by the ball.

The core may be formed using a rubber composition which includes a co-crosslinking agent, an organic peroxide, an inert filler and an organosulfur compound. It is preferable to use polybutadiene as the base rubber in the rubber composition.

It is desirable for the polybutadiene serving as the rubber component to have a cis-1,4-bond content on the polymer chain of at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. Too low a cis-1,4-bond content among the bonds on the molecule may result in a lower resilience.

Moreover, the polybutadiene has a 1,2-vinyl bond content on the polymer chain of typically not more than 2%, preferably not more than 1.7%, and more preferably not more than 1.5%. Too high a 1,2-vinyl bond content may result in a lower resilience.

To obtain a molded and vulcanized rubber composition of good resilience, the polybutadiene used in the invention is preferably one synthesized with a rare-earth catalyst or a Group VIII metal compound catalyst. Polybutadiene synthesized with a rare-earth catalyst is especially preferred.

Such rare-earth catalysts are not subject to any particular limitation. Exemplary rare-earth catalysts include those made up of a combination of a lanthanide series rare-earth compound with an organoaluminum compound, an alumoxane, a halogen-bearing compound and an optional Lewis base.

Examples of suitable lanthanide series rare-earth compounds include halides, carboxylates, alcoholates, thioalcoholates and amides of atomic number 57 to 71 metals.

In the practice of the invention, the use of a neodymium catalyst in which a neodymium compound serves as the lanthanide series rare-earth compound is particularly advantageous because it enables a polybutadiene rubber having a high cis-1,4 bond content and a low 1,2-vinyl bond content to be obtained at an excellent polymerization activity. Suitable examples of such rare-earth catalysts include those mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A 2002-293996.

To enhance the resilience, it is preferable for the polybutadiene synthesized using the lanthanide series rare-earth compound catalyst to account for at least 10 wt %, preferably at least 20 wt %, and more preferably at least 40 wt %, of the rubber components.

Rubber components other than the above-described polybutadiene may be included in the base rubber, insofar as the objects of the invention are attainable. Illustrative examples of rubber components other than the above-described polybutadiene include other polybutadienes, and other diene rubbers, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.

Examples of co-crosslinking agents include unsaturated carboxylic acids and the metal salts of unsaturated carboxylic acids.

Specific examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The metal salts of unsaturated carboxylic acids, while not subject to any particular limitation, are exemplified by the above-mentioned unsaturated carboxylic acids neutralized with a desired metal ion. Specific examples include the zinc and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

The unsaturated carboxylic acid and/or metal salt thereof is included in an amount, per 100 parts by weight of the base rubber, of generally at least 10 parts by weight, preferably at least 15 parts by weight, and more preferably at least 20 parts by weight, but generally not more than 60 parts by weight, preferably not more than 50 parts by weight, more preferably not more than 45 parts by weight, and most preferably not more than 40 parts by weight. Too much may make the core too hard, giving the ball an unpleasant feel on impact, whereas too little may lower the rebound.

The organic peroxide may be a commercially available product, suitable examples of which include Percumyl D (produced by NOF Corporation), Perhexa 3M and Perhexa C-40 (NOF Corporation), and Luperco 231XL (Atochem Co.). These may be used singly or as a combination of two or more thereof.

The amount of organic peroxide included per 100 parts by weight of the base rubber is generally at least 0.1 part by weight, preferably at least 0.3 part by weight, more preferably at least 0.5 part by weight, and most preferably at least 0.7 part by weight, but generally not more than 5 parts by weight, preferably not more than 4 parts by weight, more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. Too much or too little organic peroxide may make it impossible to achieve a ball having a good feel on impact, durability and rebound.

Examples of suitable inert fillers include zinc oxide, barium sulfate and calcium carbonate. These may be used singly or as a combination of two or more thereof.

The amount of inert filler included per 100 parts by weight of the base rubber is generally at least 1 part by weight, and preferably at least 5 parts by weight, but generally not more than 50 parts by weight, preferably not more than 45 parts by weight, more preferably not more than 40 parts by weight, and most preferably not more than 35 parts by weight. Too much or too little inert filler may make it impossible to achieve a proper weight and a good rebound.

In addition, an antioxidant may be included if necessary. Illustrative examples of suitable commercial antioxidants include Nocrac 200, Nocrac NS-6, Nocrac NS-30 (all available from Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (available from Yoshitomi Pharmaceutical Industries, Ltd.). These may be used singly or as a combination of two or more thereof.

The amount of antioxidant included per 100 parts by weight of the base rubber is generally 0 or more part by weight, preferably at least 0.05 part by weight, and more preferably at least 0.1 part by weight, but generally not more than 3 parts by weight, preferably not more than 2 parts by weight, more preferably not more than 1 part by weight, and most preferably not more than 0.5 part by weight. Too much or too little antioxidant may make it impossible to achieve a good rebound and durability.

To enhance the rebound of the golf ball and increase its initial velocity, it is preferable to include within the core an organosulfur compound.

No particular limitation is imposed on the organosulfur compound, provided it improves the rebound of the golf ball. Exemplary organosulfur compounds include thiophenols, thionaphthols, halogenated thiophenols, and metal salts thereof. Specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, the zinc salt of pentachlorothiophenol, the zinc salt of pentafluorothiophenol, the zinc salt of pentabromothiophenol, the zinc salt of p-chlorothiophenol; and diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having 2 to 4 sulfurs. The zinc salt of pentachlorothiophenol is especially preferred.

It is recommended that the amount of the organosulfur compound included per 100 parts by weight of the base rubber be generally at least 0.05 part by weight, and preferably at least 0.1 part by weight, but generally not more than 5 parts by weight, preferably not more than 4 parts by weight, more preferably not more than 3 parts by weight, and most preferably not more than 2.5 parts by weight. If too much organosulfur compound is included, the effects of addition may peak so that further addition has no apparent effect, whereas the use of too little organosulfur compound may fail to confer the effects of such addition to a sufficient degree.

Next, the outermost cover layer in the invention is described.

In the present invention, the outermost cover layer is formed primarily of a non-ionomeric resin. The non-ionomeric material is preferably a thermoplastic resin selected from among polyester elastomers, polyamide elastomers, polyurethane elastomers and mixtures thereof. A polyurethane elastomer is most preferred.

The polyurethane elastomer used as the outer cover layer material is not subject to any particular limitation, although the use of a thermoplastic polyurethane is preferable in terms of amenability to mass production. In the present invention, it is preferable to use a cover molding material (C) composed primarily of the following components A and B:

-   (A) a thermoplastic polyurethane material; and -   (B) an isocyanate mixture of (b-1) an isocyanate compound having at     least two isocyanate groups as functional groups per molecule,     dispersed in (b-2) a thermoplastic resin which is substantially     non-reactive with isocyanate.

In the practice of the invention, when the outermost cover layer is made of the above cover molding material (C), a golf ball having a better feel, controllability, cut resistance, scuff resistance and durability to cracking on repeated impact can be obtained.

Components A, B and C are described below.

(A) Thermoplastic Polyurethane Material

The thermoplastic polyurethane material has a morphology which includes soft segments composed of a polymeric polyol (polymeric glycol) and hard segments composed of a chain extender and a diisocyanate. The polymeric polyol used as a starting material may be any that has hitherto been employed in the art relating to thermoplastic polyurethane materials, without particular limitation. Exemplary polymeric polyols include polyester polyols and polyether polyols, although polyether polyols are better than polyester polyols for synthesizing thermoplastic polyurethane materials that provide a high rebound resilience and have excellent low-temperature properties. Suitable polyether polyols include polytetramethylene glycol and polypropylene glycol. Polytetramethylene glycol is especially preferred for achieving a good rebound resilience and good low-temperature properties. The polymeric polyol has an average molecular weight of preferably 1,000 to 5,000. To synthesize a thermoplastic polyurethane material having a high rebound resilience, an average molecular weight of 2,000 to 4,000 is especially preferred.

Preferred chain extenders include those used in the prior art relating to thermoplastic polyurethane materials. Illustrative, non-limiting, examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol, and 2,2-dimethyl-1,3-propanediol. These chain extenders have an average molecular weight of preferably 20 to 15,000.

Diisocyanates suitable for use include those employed in the prior art relating to thermoplastic polyurethane materials. Illustrative, non-limiting, examples include aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. Depending on the type of isocyanate used, the crosslinking reaction during injection molding may be difficult to control. In the present invention, to ensure stable reactivity with the subsequently described isocyanate mixture (B), it is most preferable to use an aromatic diisocyanate, and specifically 4,4′-diphenylmethane diisocyanate.

A commercial product may be suitably used as the above-described thermoplastic polyurethane material. Illustrative examples include Pandex T-8290, Pandex T-8295 and Pandex T-8260 (all manufactured by DIC Bayer Polymer, Ltd.), and Resamine 2593 and Resamine 2597 (both manufactured by Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.).

(B) Isocyanate Mixture

The isocyanate mixture (B) is prepared by dispersing (b-1) an isocyanate compound having as functional groups at least two isocyanate groups per molecule in (b-2) a thermoplastic resin that is substantially non-reactive with isocyanate. Above isocyanate compound (b-1) is preferably an isocyanate compound used in the prior art relating to thermoplastic polyurethane materials. Illustrative, non-limiting, examples include aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. From the standpoint of reactivity and work safety, the use of 4,4′-diphenylmethane diisocyanate is most preferred.

The thermoplastic resin (b-2) is preferably a resin having a low water absorption and excellent compatibility with thermoplastic polyurethane materials. Illustrative, non-limiting, examples of such resins include polystyrene resins, polyvinyl chloride resins, ABS resins, polycarbonate resins and polyester elastomers (e.g., polyether-ester block copolymers, polyester-ester block copolymers). From the standpoint of rebound resilience and strength, the use of a polyester elastomer, particularly a polyether-ester block copolymer, is especially preferred.

In the isocyanate mixture (B), it is desirable for the relative proportions of the thermoplastic resin (b-2) and the isocyanate compound (b-1), expressed as the weight ratio (b-2):(b-1), to be from 100:5 to 100:100, and especially from 100:10 to 100:40. If the amount of the isocyanate compound (b-1) relative to the thermoplastic resin (b-2) is too small, a greater amount of the isocyanate mixture (B) will have to be added to achieve an amount of addition sufficient for the crosslinking reaction with the thermoplastic polyurethane material (A). As a result, the thermoplastic resin (b-2) will exert a large influence, compromising the physical properties of the cover-molding material (C). On the other hand, if the amount of the isocyanate compound (b-1) relative to the thermoplastic resin (b-2) is too large, the isocyanate compound (b-1) may cause slippage to occur during mixing, making preparation of the isocyanate mixture (B) difficult.

The isocyanate mixture (B) can be obtained by, for example, adding the isocyanate compound (b-1) to the thermoplastic resin (b-2) and thoroughly working together these components at a temperature of 130 to 250° C. using mixing rolls or a Banbury mixer, then either pelletizing or cooling and subsequently grinding. A commercial product such as Crossnate EM30 (made by Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.) may be suitably used as the isocyanate mixture (B).

(C) Cover-Molding Material The cover-molding material (C) is composed primarily of the above-described thermoplastic polyurethane material (A) and isocyanate mixture (B). The relative proportion of the thermoplastic polyurethane material (A) to the isocyanate mixture (B) in the cover-molding material (C), expressed as the weight ratio (A):(B), is preferably from 100:1 to 100:100, more preferably from 100:5 to 100:50, and even more preferably from 100:10 to 100:30. If too little isocyanate mixture (B) is included with respect to the thermoplastic polyurethane material (A), a sufficient crosslinking effect will not be achieved. On the other hand, if too much is included, unreacted isocyanate may discolor the molded material.

In addition to the above-described ingredients, other ingredients may be included in the cover-molding material (C). For example, thermoplastic polymeric materials other than the thermoplastic polyurethane material may be included; illustrative examples include polyester elastomers, polyamide elastomers, ionomer resins, styrene block elastomers, polyethylene and nylon resins. Thermoplastic polymeric materials other than the thermoplastic polyurethane material may be included in an amount of 0 to 100 parts by weight, preferably 10 to 75 parts by weight, and more preferably 10 to 50 parts by weight, per 100 parts by weight of the thermoplastic polyurethane material serving as the essential component. The amount of such thermoplastic polymeric materials used is selected as appropriate for such purposes as adjusting the hardness of the cover material, improving the rebound, improving the flow properties, and improving adhesion. If necessary, various additives such as pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and parting agents may also be suitably included in the cover-molding material (C).

Formation of the cover from the cover-molding material (C) can be carried out by adding the isocyanate mixture (B) to the thermoplastic polyurethane material (A) and dry mixing, then using an injection molding machine to mold the mixture into a cover over the core. The molding temperature varies with the type of thermoplastic polyurethane material (A), although molding is generally carried out within a temperature range of 150 to 250° C.

Reactions and crosslinking which take place in the golf ball cover obtained as described above are believed to involve the reaction of isocyanate groups with hydroxyl groups remaining in the thermoplastic polyurethane material to form urethane bonds, or the creation of an allophanate or biuret crosslinked form via a reaction involving the addition of isocyanate groups to urethane groups in the thermoplastic polyurethane material. Although the crosslinking reaction has not yet proceeded to a sufficient degree immediately after injection molding of the cover-molding material (C), the crosslinking reaction can be made to proceed further by carrying out an annealing step after molding, in this way conferring the golf ball cover with useful characteristics. “Annealing,” as used herein, refers to heat aging the cover at a constant temperature for a given length of time, or aging the cover for a fixed period at room temperature.

In addition to the above resin components, various optional additives may be included in the above-described resin material for the outermost cover layer. Such additives include, for example, pigments, dispersants, antioxidants, ultraviolet absorbers, ultraviolet stabilizers, parting agents, plasticizers, and inorganic fillers (e.g., zinc oxide, barium sulfate, titanium dioxide).

The outermost cover layer has a thickness which is at least 0.5 mm but not more than 2.0 mm, preferably at least 0.5 mm but not more than 1.5 mm, and more preferably at least 0.6 mm but not more than 1.3 mm. Moreover, the outermost cover layer has a hardness (material hardness) which, expressed as the Shore D hardness, is in a range of from 35 to 60, preferably 40 to 60, and more preferably 42 to 58. Setting the cover thickness and Shore D hardness outside of these ranges will worsen the feel of the ball on impact and the spin performance, and thus make it impossible to achieve the intended effects of the invention.

The intermediate layer disposed between the above core and the above outermost cover layer is described below.

The intermediate layer has a thickness of at least 0.5 mm but not more than 2.5 mm, preferably at least 0.8 mm but not more than 2.2 mm, and more preferably at least 1.5 mm but not more than 2.0 m. Outside of this range, the balance between the spin performance and initial velocity of the ball will be poor, resulting in a decrease in the flight performance.

The surface hardness (Shore D) of the intermediate layer, i.e., the Shore D hardness at the surface of the sphere composed of the core enclosed by the intermediate layer, while not subject to any particular limitation, is preferably at least 60 but not more than 80, more preferably at least 63 but not more than 77, and even more preferably at least 67 but not more than 73. At a hardness lower than the above range, the ball may take on too much spin on full shots, and may therefore not travel as far as desired. Moreover, the feel on impact may be too soft. On the other hand, at a hardness greater than the above range, the spin rate may decrease, making the ball more difficult to control, the feel of the ball may become too hard, and the durability of the ball to cracking on repeated impact may worsen. As used herein, “surface hardness of the intermediate layer” refers to the hardness at the surface of the sphere obtained by enclosing the core with the intermediate layer material, and is determined by such factors as the hardness of the underlying core and the thickness and hardness of the intermediate layer, and differs from the hardness of the intermediate layer material proper. Also, the intermediate layer must be harder than the surface of the outermost layer.

In the practice of the invention, it is critical for the intermediate layer material to be composed primarily of a resin material obtained by blending together (I) a sodium ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer with (II) a magnesium ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer. The impact resistance of an ionomer is generally determined by such factors as the type of cation and the resin hardness. In the material employed in the present invention, because it is known that using the sodium ion neutralization product of a random copolymer in combination with the magnesium ion neutralization product of a random copolymer enables the impact resistance and durability of the resulting golf ball to be improved to a greater extent than using such a sodium ion neutralization product by itself, above materials (I) and (II) are used in combination.

It is possible here to additionally blend another resin material, such as a random terpolymer, together with above resin materials (I) and (II). The above terpolymer may be suitably admixed within a range that allows the objects of the invention to be attained, such as a range of about 0 to 5 parts by weight per 100 parts by weight of the base resin.

It is preferable to use an α-olefin as the olefin in above component (I) or component (II). Illustrative examples of α-olefins include ethylene, propylene and 1-butene. Of these, ethylene is especially preferred. These olefins may be used in combinations of two or more thereof.

The unsaturated carboxylic acid in component (I) or component (II) is preferably an α,β-unsaturated carboxylic acid having from 3 to 8 carbons. Illustrative examples of α,β-unsaturated carboxylic acids having 3 to 8 carbons include acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, maleic acid and fumaric acid. Of these, acrylic acid and methacrylic acid are preferred. These unsaturated carboxylic acids may be used in combinations of two or more thereof.

The unsaturated carboxylic acid content in these copolymers is preferably from 5 to 20 wt %, both for component (I) and component (II). If the unsaturated carboxylic acid content is too low, the intermediate material may have a lower rigidity and resilience, possibly diminishing the flight performance of the golf ball. On the other hand, if the unsaturated carboxylic acid content is too high, the intermediate layer may lack sufficient flexibility.

When component (I) and component (II) are used in admixture, the mixing ratio therebetween by weight, expressed as (I)/(II), is preferably from 20/80 to 80/20, and more preferably from 25/75 to 75/25.

The ionomer resin used in the invention may be a commercial product, illustrative examples of which include Surlyn (produced by E.I. DuPont de Nemours & Co.) and Himilan (produced by DuPont-Mitsui Polychemicals Co., Ltd.).

The intermediate layer material has a Shore D hardness of at least 55 but not more than 70, and preferably at least 58 but not more than 65.

The golf ball of the invention can be manufactured using an ordinary process such as a known injection molding process to form on top of one another the respective layers described above--the core, intermediate layer, and cover. For example, a molded and vulcanized article composed primarily of the rubber material may be placed as the core within a particular injection-molding mold, following which the intermediate layer material may be injection-molded over the core to give an intermediate spherical body. The spherical body may then be placed within another injection-molding mold and the cover material injection-molded over the spherical body to give a multi-piece golf ball. Alternatively, the cover may be formed as a layer over the intermediate spherical body by, for example, placing two half-cups, molded beforehand as hemispherical shells, around the intermediate spherical body so as to encase it, then molding under applied heat and pressure.

Numerous dimples may be formed on the surface of the cover. The dimples arranged on the cover surface, while not subject to any particular limitation, number preferably at least 250 but not more than 500, more preferably at least 280 but not more than 360, and even more preferably at least 300 but not more than 350. If the number of dimples is higher than the above range, the ball will tend to have a low trajectory, which may shorten the distance of travel. On the other hand, if the number of dimples is too small, the ball will tend to have a high trajectory, as a result of which an increased distance may not be achieved.

Any one or combination of two or more dimple shapes, including circular shapes, various polygonal shapes, dewdrop shapes and oval shapes, may be suitably used. If circular dimples are used, the diameter of the dimples may be set to at least about 2.5 mm but not more than about 6.5 mm, and the depth may be set to at least 0.08 mm but not more than 0.30 mm.

To fully manifest the aerodynamic characteristics of the dimples, the dimple coverage on the spherical surface of the golf ball, which is the sum of the individual dimple surface areas, each defined by the border of the flat plane circumscribed by the edge of the dimple, expressed as a ratio (SR) with respect to the spherical surface area of the ball were it to be free of dimples, is preferably at least 60% but not more than 90%. Also, to optimize the trajectory of the ball, the value V0 obtained by dividing the spatial volume of each dimple below the flat plane circumscribed by the edge of that dimple by the volume of a cylinder whose base is the flat plane and whose height is the maximum depth of the dimple from the cylinder base is preferably at least 0.35 but not more than 0.80. In addition, the VR value, which is the sum of the volumes of individual dimples formed below flat planes circumscribed by the dimple edges, as a percentage of the volume of the ball sphere were it to have no dimples thereon, is preferably at least 0.6% but not more than 1.0%. Outside of the above ranges for these values, the ball may assume a trajectory that is not conducive to achieving a good distance, as a result of which the ball may fail to travel a sufficient distance when played.

The golf ball of the invention may be manufactured so as to conform with the Rules of Golf for competitive play. That is, it may be produced to a ball diameter which is of a size that will not pass through a ring having an inside diameter of 42.672 mm, but is not more than 42.80 mm, and to a weight of generally from 45.0 to 45.93 g.

As explained above, the golf ball of the invention has a flight performance and controllability acceptable for use by professional golfers and skilled amateurs, and also has an excellent durability to cracking on repeated impact.

EXAMPLES

Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation.

Examples 1 to 3, Comparative Examples 1 and 2

Using the core materials composed primarily of polybutadiene shown in Table 1 below, solid cores having a diameter of 37.3 mm, a weight of 31.9 g and a deflection (deformation) of 4.1 mm were produced. The deflection values were measured as the amount of deformation experienced by the core when it was compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf).

TABLE 1 Amount formulated (pbw) Formulation Polybutadiene (1) 80 Polybutadiene (2) 20 Zinc oxide 21.3 Zinc stearate 5.0 Zinc salt of 1.5 pentachlorothiophenol Antioxidant 0.1 Zinc acrylate 32.0 Peroxide 3.0 Sulfur 0.1 Specific gravity 1.2 Specifications Diameter (mm) 37.3 Weight (g) 31.9 Deflection (mm) 4.1

Details concerning the above formulation are given below.

-   Butadiene rubber (1): Produced by JSR Corporation under the trade     name BR 730. -   Butadiene rubber (2): Produced by JSR Corporation under the trade     name BR 51. -   Zinc stearate: Produced by NOF Corporation under the trade name Zinc     Stearate G. -   Antioxidant: Produced by Sumitomo Chemical Co., Ltd. under the trade     name ANTIGENE BH-T. -   Peroxide: Dicumyl peroxide. Produced by NOF Corporation under the     trade name Percumyl D.

Next, an intermediate layer material of the composition shown in Table 3 was injection-molded to a thickness of 1.67 mm within a mold in which the above solid core had been placed. The cover material shown in Table 2 was then injection-molded to a thickness of 1.01 mm over the intermediate layer material-enclosed core within another mold, thereby producing a three-piece solid golf ball having a diameter of 42.7 mm. The intermediate layer material was prepared by mixture at 200° C. in a co-rotating, intermeshing twin-screw extruder (screw diameter, 32 mm; L/D=30; main motor output, 7.5 kw; with vacuum vent).

TABLE 2 Amount formulated (pbw) Formulation T-8295 50 T-8290 50 Titanium oxide 3.8 Polyethylene wax 1.4 Isocyanate compound 18 Specific gravity 1.01 Weight (g) 5.78 Material hardness (Shore D hardness) 48

Details concerning the above formulation are given below.

-   T-8290, T-8295: MDI-PTMG type thermoplastic polyurethanes produced     by DIC Bayer Polymer under the trademark designation Pandex. -   Titanium oxide: Produced by Ishihara Sangyo Kaisha, Ltd. under the     trade name Tipaque R550. -   Polyethylene wax: Produced by Sanyo Chemical Industries, Ltd. under     the trade name Sanwax 161P.

Isocyanate Compound:

Crossnate EM30 (trade name), an isocyanate masterbatch which is produced by Dainichi Seika Colour & Chemicals Mfg. Co., Ltd., contains 30% of 4,4′-diphenylmethane diisocyanate (measured concentration of amine reverse-titrated isocyanate according to JIS-K1556, 5 to 10%), and in which the masterbatch base resin is a polyester elastomer (Hytrel 4001, produced by DuPont-Toray Co., Ltd.). The isocyanate compound was mixed at the time of injection molding.

Details concerning the above formulation are given below.

-   Himilan 1706 (trade name): An ionomer resin which is a zinc     ion-neutralized ethylene-methacrylic acid random copolymer produced     by DuPont-Mitsui Polychemicals Co., Ltd. -   Himilan 1605 (trade name): An ionomer resin which is a sodium     ion-neutralized ethylene-methacrylic acid random copolymer produced     by DuPont-Mitsui Polychemicals Co., Ltd. -   AM7311 (trade name): An ionomer resin which is a magnesium     ion-neutralized ethylene-methacrylic acid random copolymer produced     by DuPont-Mitsui Polychemicals Co., Ltd. -   TMP (trade name): A trimethylolpropane produced by Mitsubishi Gas     Chemical Co., Ltd.

TABLE 3 Comparative Example Example 1 2 3 1 2 Intermediate H1706 (Zn ion type) 100 layer resin H1605 (Na ion type) 30 50 70 100 formulation AM7311 (Mg ion type) 70 50 30 TMP 1.1 1.1 1.1 1.1 1.1 Resin MFR (190° C., g/10 min) 1.3 1.7 1.5 0.7 2.0 properties Specific gravity 0.94 0.94 0.95 0.96 0.95 Shore D hardness 65 65 66 62 64 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 properties Weight (g) 45.6 45.6 45.6 45.6 45.6 Deflection hardness (mm) 2.8 2.7 2.7 2.9 2.8 Initial velocity (m/s) 77.0 77.1 77.1 76.6 76.9 Durability to cracking 170 178 171 140 143 on repeated impact (shot number) Note: Numbers for the intermediate layer resin formulation are shown as parts by weight.

Evaluation of Cover Material Properties

-   -   Melt Mass Flow Rate:

The melt mass flow rate of a material measured in accordance with JIS-K6760 (test temperature, 190° C.; test load, 21 N (2.16 kgf)).

-   -   Cover Resin Hardness:

The shore D hardness measured in accordance with ASTM D-2240 is shown.

Evaluation of Ball Properties

-   -   Ball Deflection (Deformation) (mm):

The deformation (mm) of the golf ball when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was determined.

-   -   Ball Initial Velocity (m/s):

The initial velocity (m/s) was measured using an initial velocity measuring apparatus of the same type as that of the official golf ball regulating-body—R&A (USGA), and in accordance with R&A (USGA) rules.

-   -   Durability to Repeated Impact:

The golf ball was repeatedly struck at a head speed (HS) of 45 m/s, and the durability to repeated impact was rated as the number of times the ball had been hit when the rebound decreased successively by 3%. Each value shown in the table is the average shot number for four balls. 

1. A golf ball comprising a core, an outermost cover layer and an intermediate layer therebetween, wherein the intermediate layer is formed primarily of a resin material obtained by blending together (I) a sodium ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer with (II) a magnesium ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer, the intermediate layer-forming resin material having a Shore D hardness of from 55 to 70 and the intermediate layer having a thickness of from 0.5 to 2.5 mm; and the outermost cover layer is formed primarily of a non-ionomeric resin material, the cover-forming resin material having a Shore D hardness of from 35 to 60 and the cover having a thickness of from 0.5 to 2.0 mm.
 2. The golf ball of claim 1, wherein the intermediate layer-forming material contains material (I) and material (II) in a mixing ratio by weight of from 20/80 to 80/20.
 3. The golf ball of claim 1, wherein the non-ionomeric resin material in the outermost cover layer is a thermoplastic polyurethane elastomer. 